1. Field of the Invention
The present invention relates to a color image read apparatus reading an original image by decomposing it into plural color signals by photoelectric conversion, and to be further detailed, relates to a color image read apparatus which performs data corrections such as shading correction and color balance correction and binary coding for image data having been read.
2. Description of the Prior Art
A color image read apparatus reads a color image of an original by decomposing it into the three primary colors of light (red(R), green(G) and blue(B)) using a CCD sensor which is an optoelectric transducer and filters. Then it converts analog outputs of the respective colors of the linear CCD sensor into digital values using A/D converters, subsequently makes shading correction for correcting nonuniformity of the quantity of light irradiated on one line of the original and dispersion of the sensitivities of CCD chips configurating a linear CCD sensor and gamma correction for producing an input-output relation matching with characteristics of a data outputting apparatus, and thereafter performs binary coding by a predetermined system. The color image thus read is sent to a data outputting apparatus (for example, a printer or CRT).
Generally, in the color image read apparatus, the spectral distribution characteristics of a light source (for example, a fluorescent lamp) used for scanning of an original are not the same in balance of the light intensities at the respective wavelengths of the three primary colors of light, R, G and B. Also, the filters for R, G and B (for example, vacuum-evaporated on the surface of a linear CCD sensor being an optoelectric transducer) have not the same sensitivity to the respective R, G and B. For this reason, even if a white pattern having a certain density is read, the respective signal outputs of R, G and B are not uniform, for example, resulting in ratios of R: G: B=5: 4: 3. Therefore correction for color balance is required to be made.
Conventionally, the correction of color balance has been made using a shading correcting circuit with differences in the above-described signal outputs taken as dispersion of picture elements. In this case, the system of making correction for every data of one bit of the optoelectric transducer inputted sequentially in the order of R, G and B and the system provided with shading correcting circuits for the respective three primary colors R, G and B are generally employed.
In the system which makes a correction of color balance using one shading correcting circuit, the range of shading correction is required to be set so as to be able to correct up to a signal output of about 30-40% of a maximum signal output, and therefore the reliability of data shading-corrected is reduced remarkably. Also, only one shading correcting circuit is used for all signals of R, G and B, and correction data to the respective colors R, G and B have to be put beforehand in a table of a ROM in the shading correcting circuit. This means that a deficiency is produced that a large capacity of data is required to be stored in advance.
Also, the system using three shading correcting circuits has a deficiency that the scale of the circuit is made larger.